Radio frequency identification technology, sometimes referred to as RFID technology, has a variety of commercial applications, and is typically used for object identification and tracking.
This section describes at least one embodiment of a typical radio frequency identification (“RFID”) tag and reader, this embodiment and others are well known in the art. FIG. 1 illustrates a typical radio frequency identification (“RFID”) tag 10. The RFID tag 10 includes a substrate 12 having a first major surface 14 and a second major surface 16 opposite the first major surface 14. The substrate 12 may optionally be a flexible substrate, such that it could be used in a label that may be wrapped around an object. The flexible substrate 12 could have enough flexibility to conform to a variety of surfaces and bend easily around objects. For example, the substrate 12 may be in the range of 25-100 microns in thickness, and may be made of a flexible material, such as polyester, polyethylene naphthanate, polyimide, polypropylene, paper, or other flexible materials apparent to those skilled in the art.
An RFID element is attached to the first major surface 14 of the substrate 12. The RFID element has information storage and transmission capabilities adapted to enable an interrogation system to obtain information from the radio frequency-responsive element. The RFID element typically includes two major components: an integrated circuit 20 and an antenna 18. The integrated circuit 20 provides the primary identification function. It includes circuitry to permanently store the tag identification and other desirable information, interpret and process commands received from the interrogation hardware, respond to requests for information by the interrogator, and assist the hardware in resolving conflicts resulting from multiple tags responding to interrogation simultaneously. Optionally, the integrated circuit may provide for updating the information stored in its memory (read/write) as opposed to just reading the information out (read only). Some exemplary integrated circuits suitable for use in RFID tags 10 include those commercially available from Texas Instruments™ (in their line of products under the trade names TI-RFid™ or TAG-IT™), Philips and/or NXP Electronics Co. (in their line of products under the trade names I-CODE™, MIFARE™, and HITAG™), among others.
The antenna 18 geometry and properties depend on the desired operating frequency of the RFID tag 20. For example, 915 MHz or 2.45 GHz RFID tags 10 would typically include a dipole antenna, such as a linear dipole antenna or a folded dipole antenna (not shown). A 13.56 MHz (or similar) RFID tag 10 would typically use a spiral or coil antenna 18, as shown in FIG. 1. However, other antenna designs are known to those skilled in the art. The antenna 18 intercepts the radio frequency energy radiated by an interrogation source, such as a RFID reader 60 illustrated schematically in FIG. 2. (Reference number 62 illustrates the radio frequency energy radiated by the RFID reader 60.) Radio frequency energy 62 carries both power and commands to the tag 10. The antenna enables the RF-responsive element to absorb energy sufficient to power the integrated circuit 20 and thereby provide the response to be detected. Thus, the characteristics of the antenna are typically matched to the system in which it is incorporated. In the case of tags operating in the high MHz to GHz range, one of the most important characteristics is typically the antenna size. Often, the effective length of a dipole antenna is selected so that it is close to a half wavelength or multiple half wavelength of the interrogation signal. In the case of tags operating in the low to mid MHz region (13.56 MHz, for example) where a half wavelength antenna is impractical due to size limitations, the important characteristics are typically antenna inductance and the number of turns on the antenna coil. Often, metals such as copper or aluminum are used, but other conductors, including printed inks, are also acceptable. It is also important that the input impedance of the selected integrated circuit match the impedance of the antenna for maximum energy transfer. Additional information about antennas is known to those of ordinary skill in the art, for example, in reference texts such as RFID Handbook, Radio-Frequency Identification Fundamentals and Applications, by K. Finkenzeller, (1999 John Wiley & Sons Ltd, Chichester, West Sussex, England).
A capacitor 22 is often included to increase the performance of the RFID tag 10. The capacitor 22, when present, aids in tuning the operating frequency of the tag to a particular value. This is desirable for obtaining maximum operating range. The capacitor may either be a discrete component or may be integrated into the antenna 18.
An RFID reader or interrogator 60 is schematically illustrated in FIG. 2. The RFID reader 60 includes an RFID reader antenna 64. RFID readers 60 are well known in the art. For example, commercially available RFID readers are available from 3M Company based in St. Paul sold under the trade name 3M™ Digital Library Assistant™ as model numbers 702, 703, 802, and 803. Another example of a commercially available RFID reader is a model IP4 portable RFID (UHF) Reader attached to an Intermec™ 700 Series Mobile computer available from Intermec Technologies Corporation, Everett, Wash.
The RFID reader 60 and RFID tag 10 form an RFID system. Inductively coupled RFID systems are based on near-field magnetic coupling between the antenna loop of the RFID reader and the antenna coil of the RFID transponder, according to RFID Handbook, Radio-Frequency Identification Fundamentals and Applications, by K. Finkenzeller, (1999 John Wiley & Sons Ltd, Chichester, West Sussex, England) pp. 21. A number of RFID systems are available, following one of several communication and system performance standards. The discussion below is principally based on RFID systems operating at 13.56 MHz, but the discussion extends to inductively coupled RFID systems at other operating frequencies and provides insights into the interference that conductive objects can pose to electromagnetically coupled RFID systems.
Radio frequency-responsive tags can be either active or passive. An active tag incorporates an additional energy source, such as a battery, into the tag construction. This energy source permits active radio frequency-responsive tags to create and transmit strong response signals even in regions where the interrogating radio frequency field is weak, and thus an active radio frequency-responsive tag can be detected at greater range. However, the relatively short lifetime of the battery limits the useful life of the tag. In addition, the battery adds to the size and cost of the tag. A passive tag derives the energy needed to power the tag from the interrogating radio frequency field, and uses that energy to transmit response codes by modulating the impedance the antenna presents to the interrogating field, thereby modulating the signal reflected back to the reader antenna. Thus, their range is more limited. Because passive tags are preferred for many applications, the remainder of the discussion will be confined to this class of tag. Those skilled in the art, however, will recognize that these two types of tags share many features and that both can be used in the examples of this disclosure.